This invention relates generally to a mirror, and more particularly to a concave mirror used for reflecting and focusing an image of the sun in a solar energy collection system.
In one typical solar energy collection system, a large number of mirrors are arranged in an array with each mirror focused on a receiver that converts the reflected radiant energy of the sun into heat or electrical energy. It is desirable to use mirrors which are concave, or spherically curved, so that the reflected image of the sun can be concentrated within a relatively small area on the receiver. Rectangular mirrors are advantageous over round mirrors since when arranged to form an array, rectangular mirrors provide more reflective surface per unit area of the array. Also, more waste results from the production of a round mirror as compared to a rectangular mirror, since round mirrors are cut from rectangular sheets.
Generally, the mirrors used in such applications are formed from a flat sheet of glass having a silvered back surface. To reduce energy losses caused by the transmission of the sun's rays through the glass of the mirror, the mirror is very thin and, as a result, flexible. Thus, the mirror must be supported from behind in order for it to consistently maintain a concave shape which accurately focuses solar radiation onto the receiver. Previously, this support has been provided by a glass backing element which is thicker and more rigid than the mirror. The backing element is formed with a concave depression into which the flexible mirror is pressed to assume a concave shape. The abutting surfaces of the mirror and the backing element are laminated together with adhesive so that the two sheets of glass form a single laminated structure.
This lamination technique has suffered from several drawbacks, one being that forming the depression in the backing element requires a time consuming and costly machining procedure. Also, once the mirror is laminated to the backing element, the degree of curvature of the mirror cannot be adjusted. Thus, the depression must be precisely machined and the adhesive properly distributed over the depression or else the mirror may not be usable for its intended purpose.
Further, since the mirror and the glass backing element form a single, relatively thick structure, the laminated assembly has a greater moment of inertia, and thus is more rigid than either piece of glass separately. As a result, the assembly can withstand only a relatively small degree of deformation before maximum stress limits are reached, potentially causing breakage of the assembly when it is deformed by wind or thermal expansion. Stresses induced during shipment of the assembly to the construction site of the array may also cause breakage. Due to the precision required for laminating the mirror to the backing element, it is impractical to ship the mirror and backing element separately and assemble them at the construction site.
To minimize stress build-ups and potential breakage, the size of laminated mirror assemblies generally must be limited. As a result, a relatively large number of mirrors must be utilized to obtain a given amount of reflective surface area for an array. This is undesirable since the cost of constructing an array is dependent on the number of mirrors used.
Thus, a need exists for a non-laminated, preferably rectangular mirror having a concave curvature.